1. Field of the Invention
The present invention relates to a method and system for uplink power control, and more particularly to a method and system for uplink power control when a Hybrid Automatic Repeat reQuest (HARQ) scheme is used in a communication system.
2. Description of the Related Art
In general, a base station and a subscriber station perform uplink power control in a cellular communication system in order to simultaneously maintain stable wireless link performance and minimize the effects of interference. For example, in an Institute of Electrical and Electronics Engineers (IEEE) 802.16e communication system, uplink power control is performed in such a manner as to maintain an uplink Carrier to Interference and Noise Ratio (CINR) to constantly hold an uplink target Packet Error Rate (PER). For example, when the uplink target PER is set to a lower value, the reliability of a wireless link increases, but an uplink CINR value required for maintaining the lower target PER also increases.
In contrast, when the uplink target PER is set to a higher value, the reliability of a wireless link decreases, but an uplink CINR value required for maintaining the higher target PER also decreases.
As a required uplink CINR value changes, the output of the subscriber station changes, so that the amount of uplink interference, capacity, and coverage area of the base station also changes. In other words, when a required uplink CINR value is great, the output of the subscriber station increases and the amount of uplink interference of the base station also increases, thereby decreasing the uplink coverage also and capacity of the base station.
In contrast, when a required uplink CINR value is small, the output of the subscriber station decreases and the amount of uplink interference of the base station also decreases, thereby increasing the uplink coverage area and capacity of the base station. Accordingly, the reliability of a wireless link secured by uplink power control has a trade-off relation with the uplink capacity and coverage of the base station.
In the IEEE 802.16e communication system, the HARQ scheme is employed in order to increase the reliability of a wireless link. When the HARQ scheme is employed, even if an error occurs in the transmission/reception of a packet, the packet can be normally transmitted/received by re-transmitting the packet in which an error has occurred. Therefore, when the HARQ scheme is employed, the uplink power control can be performed at a higher target PER, as compared with a case where the HARQ scheme is not employed. When the uplink power control can be performed at a higher target PER, as described above, a required uplink CINR value decreases, thereby decreasing the transmission power of the subscriber station, so that it is possible to increase the coverage area and capacity of the base station.
In addition, although the subscriber station uses the same transmission power, the subscriber station can operate in a higher Modulation Product Coding (MPC) when using the HARQ scheme, thereby increasing an uplink data rate. In this case, a generated gain will be referred to as a “HARQ gain.”
Although the HARQ scheme is employed in the IEEE 802.16e communication system, the HARQ scheme is not utilized with respect to management messages and part of data traffic. Data traffic for which the HARQ scheme is not used includes, for example, real-time data traffic, in which traffic for which the HARQ scheme is not used will be referred to as “non-HARQ traffic.” Therefore, traffic of one subscriber station may include both HARQ traffic and non-HARQ traffic therein. In the IEEE 802.16e communication system, a base station uses a power control Information Element (IE) within an UpLink MAP (UL MAP) message in order to control the power of a subscriber station. The format of the power control IE is defined as shown in Table 1 below.
TABLE 1SyntexsizeNotesUL-MAP_IE( )CID16 bit UIUC 4 bit if(UIUC=15)  Extended UIUC dependent IE   Power Control IE    Extended UIUC 4 bitsFast power control = 0x00    Length 4 bitsLenth = 0x01    Power Control 8 bitsSigned integer whichexpresses the change inpower level (in 0.25 dBunits) that the SS shouldapply to correct its currenttransmission power
As shown in Table 1, the power control IE is contained in an UP MAP message, so that the power control IE is identified only by a basic connection identifier (CID) contained in the UL MAP message.
In order to increase a HARQ gain in the IEEE 802.16e communication system, it is necessary to use a high target PER of about 10 to 30%. However, when the HARQ scheme is not used, it is impossible to recover an error occurring in a wireless link. Therefore, the uplink power control for non-HARQ traffic is performed at a low target PER of less than 1% in order to ensure the reliability of the wireless uplink in the IEEE 802.16e communication system. As a result, a target-PER mismatch phenomenon between HARQ traffic and non-HARQ traffic occurs. The reliability of non-HARQ traffic cannot be guaranteed if a target PER suitable to HARQ traffic is applied, while a HARQ gain cannot be guaranteed if a target PER suitable to non-HARQ traffic is applied.
However, in the IEEE 802.16e communication system, the uplink power control is performed without taking into consideration whether the HARQ scheme is used, as described with reference to Table 1. Therefore, there is an earnest need for a way to control uplink power by considering both of a case where the HARQ scheme is used and a case where the HARQ scheme is not used.